EchoScan: a new preventive tool for hearing loss prevention

Human and animal studies have shown that noise and ototoxic agents such as aromatic solvents can cause hearing loss or can exacerbate noise-induced hearing loss. The synergistic effects are partly due to a modification of the efferent reflex strength (middle- and the inner-ear acoustic reflexes).
Today, the reference screening tool for assessing traumatic (noise)- or ototoxic (chemical agent)- induced hearing loss (HL) is the pure-tone air-conduction audiometry (PTA). But this hearing test relies on individuals’ ability to determine thresholds of hearing sensations to frequency-specific acoustic stimuli and therefore allows only subjective tests to be performed. In this cognitive process, the auditory central system analyzes each piece of information coming from the peripheral auditory receptor, and, in the meantime, might counterbalance subtle ear dysfunctions or metabolic fatigue to assure the highest level of performance. Thus, HL may remain underestimated due to potential retro-cochlear compensation mechanisms, which may postpone the awareness of changes in hearing capability.
Since twenty years, we have known that outer hair cells (OHCs) are one of the main targets of both acoustic injuries (Hamernik et al., 1989) and ototoxic agents (Campo et al., 2013). Moreover, OHCs are the key element of a mechanical feedback loop at the origin of the so-called “cochlear amplifier” (Davis, 1983). Any disruption of OHC function, either by acoustic overexposure or by any OHC-specific pathology, leads to worsen the auditory performances.
The measurement of cubic-difference distortion product oto-acoustic emissions (DPOAEs) at 2f1-f2 in response to stimulations by two primaries at f1 and f2 requires the presence of healthy and motile OHCs.
DPOAEs were therefore good candidates to assess HL whose pathological origin is cochlear. By adding a contralateral acoustic stimulation to the ipsilateral DPOAE measurements (CAS DPOAEs), we have the possibility to measure a reduction in the transmission of the sound energy through the middle-ear into the cochlea and, in the meantime, to evaluate the efferent reflex strength. Taken together, CAS DPOAEs allows dysfunction of the middle-ear, inner-ear and auditory central system to be tested in one series of objective and noninvasive measures.
In this presentation, the results obtained both in animals and humans will be presented to promote the use of CAS DPOAEs for assessing the early effects of noise and/or solvents on hearing. The origin of synergistic effects on hearing of coexposure to noise and aromatic solvents will be discussed regarding the depressing effects of efferent acoustic reflexes.